This invention pertains to a novel emulsion polymerization process for the production of fluoroelastomers wherein a certain class of hydrocarbon sulfonate anionic surfactant is used as the dispersing agent.
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. Nos. 4,214,060 and 4,281,092. Generally, fluoroelastomers are produced in an emulsion polymerization process wherein a water-soluble polymerization initiator and a relatively large amount of surfactant are employed. The surfactant most often used for such processes has been ammonium perfluorooctanoate (C-8). Fluoroelastomers prepared in such processes leave the reactor in the form of a dispersion.
While C-8 works very well as a surfactant in the polymerization process, it is relatively expensive, and its future commercial availability is uncertain. Thus, it would be desirable to find other surfactants effective for use in the emulsion polymerization of fluoroelastomers.
Efforts to replace C-8 have focussed on other expensive anionic fluorosurfactants such as 1) Fxe2x80x94(CF2CF2)nCH2CH2xe2x80x94OSO3M, where n is an integer from 2-8, or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion (U.S. Pat. No. 4,524,197); 2) Fxe2x80x94(CF2CF2)nCH2CH2xe2x80x94SO3M, where n is an integer from 2-8, or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion (U.S. Pat. No. 4,380,618); and 3) C6F13CH2CH2SO3M wherein M is a cation having a valence of 1 (U.S. Pat. No. 5,688,884). Such fluorinated surfactants are effective in forming stable dispersions of highly fluorinated fluoroelastomer latex particles during the emulsion polymerization process because of the highly fluorinated structure of the surfactants.
Hydrocarbon sulfonate surfactants such as sodium dodecylbenzenesulfonate and sodium sulfosuccinate have been used in the emulsion polymerization of tetrafluoroethylene/propylene copolymers (G. Kostov et al. in God. Vissh. Kim.-Tekhnol. Inst., 29(2), 316-320 (1983)). These copolymers contain a large amount of propylene units and have a relatively low amount of fluorine, i.e. less than 58 weight percent fluorine. It is believed that hydrocarbon sulfonate surfactants can be successfully employed in the emulsion polymerization of tetrafluoroethylene/propylene copolymers because of the compatibility between the hydrocarbon surfactant and the high level of propylene units present in the copolymer. Such surfactants would be unlikely to satisfactorily stabilize highly fluorinated (i.e. having at least 58 wt. % fluorine) fluoroelastomer latex particles.
Surprisingly, it has been found that certain hydrocarbon sulfonate surfactants may be used to manufacture highly fluorinated fluoroelastomers. One aspect of the present invention provides an emulsion polymerization process for the production of fluoroelastomers, said fluoroelastomers having at least 58 weight percent fluorine, comprising:
(A) charging a reactor with a quantity of an aqueous solution comprising a surfactant of the formula CH3xe2x80x94(CH2)nxe2x80x94SO3M where n is an integer from 6 to 17, or mixtures thereof, and M is a cation having a valence of 1;
(B) charging the reactor with a quantity of a monomer mixture to form a reaction medium, said monomer mixture comprising i) from 25 to 70 weight percent, based on total weight of the monomer mixture, of a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) between 75 and 30 weight percent, based on total weight of the monomer mixture, of one or more additional copolymerizable monomers, different from said first monomer, wherein said additional monomer is selected from the group consisting of fluorine-containing olefins, fluorine-containing vinyl ethers, hydrocarbon olefins and mixtures thereof; and
(C) polymerizing said monomers in the presence of a free radical initiator to form a fluoroelastomer dispersion while maintaining said reaction medium at a pH between 1 and 7, at a pressure between 0.5 and 10 MPa, and at a temperature between 25xc2x0 C. and 130xc2x0 C.
In other aspects of the invention, the surfactant employed in (A) may be substituted by CH3xe2x80x94(CH2)nxe2x80x94C6H4xe2x80x94SO3M or CH3xe2x80x94(CH2)nxe2x80x94CHxe2x95x90CHCH2xe2x80x94SO3M, wherein n and M are as defined above.
The present invention is directed to an emulsion polymerization process for producing a fluoroelastomer. By xe2x80x9cfluoroelastomerxe2x80x9d is meant an amorphous elastomeric fluoropolymer. The fluoropolymer may be partially fluorinated or perfluorinated, so long as it contains at least 58 percent by weight fluorine, preferably at least 64 wt. % fluorine. Fluoroelastomers made by the process of this invention contain between 25 to 70 weight percent, based on the weight of the fluoroelastomer, of copolymerized units of a first monomer which may be vinylidene fluoride (VF2) or tetrafluoroethylene (TFE). The remaining units in the fluoroelastomers are comprised of one or more additional copolymerized monomers, different from said first monomer, selected from the group consisting of fluorine-containing olefins, fluorine-containing vinyl ethers, hydrocarbon olefins and mixtures thereof.
According to the present invention, fluorine-containing olefins copolymerizable with the first monomer include, but are not limited to, vinylidine fluoride, hexafluoropropylene (HFP), tetrafluoroethylene (TFE), 1,2,3,3,3-pentafluoropropene (1-HPFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.
The fluorine-containing vinyl ethers employed in the present invention include, but are not limited to perfluoro(alkyl vinyl) ethers. Perfluoro(alkyl vinyl) ethers (PAVE) suitable for use as monomers include those of the formula
CF2xe2x95x90CFO(RfO)n(RfO)mRfxe2x80x83xe2x80x83(I)
where Rf and Rf, are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A preferred class of perfluoro(alkyl vinyl) ethers includes compositions of the formula
CF2xe2x95x90CFO(CF2CFXO)nRfxe2x80x83xe2x80x83(II)
where X is F or CF3, n is 0-5, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A most preferred class of perfluoro(alkyl vinyl) ethers includes those ethers wherein n is 0 or 1 and Rf contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl) ether (PMVE) and perfluoro(propyl vinyl) ether (PPVE). Other useful monomers include compounds of the formula
CF2xe2x95x90CFO[(CF2)mCF2CFZO]nRfxe2x80x83xe2x80x83(III)
where Rf is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1, n=0-5, and Zxe2x95x90F or CF3. Preferred members of this class are those in which Rf is C3F7, m=0, and n=1.
Additional perfluoro(alkyl vinyl) ether monomers include compounds of the formula
CF2xe2x95x90CFO [(CF2CF {CF3 }O)n(CF2CF2CF2O)m(CF2)p]CxF2x+1xe2x80x83xe2x80x83(IV)
where m and n independently=0-10, p=0-3, and x=1-5.
Preferred members of this class include compounds where n=0-1, m=0-1, and x=1.
Other examples of useful perfluoro(alkyl vinyl ethers) include
CF2xe2x95x90CFOCF2CF(CF3)O(CF2O)mCnF2n+1xe2x80x83xe2x80x83(V)
where n=1-5, m=1-3, and where, preferably, n=1.
If copolymerized units of PAVE are present in fluoroelastomers prepared by the process of the invention, the PAVE content generally ranges from 25 to 75 weight percent, based on the total weight of the fluoroelastomer. If perfluoro(methyl vinyl) ether is used, then the fluoroelastomer preferably contains between 30 and 55 wt. % copolymerized PMVE units.
Hydrocarbon olefins useful in the fluoroelastomers prepared by the process of this invention include, but are not limited to ethylene (E) and propylene (P). If copolymerized units of a hydrocarbon olefin are present in the fluoroelastomers prepared by the process of this invention, hydrocarbon olefin content is generally 4 to 30 weight percent The fluoroelastomers prepared by the process of the present invention may also, optionally, comprise units of one or more cure site monomers. Examples of suitable cure site monomers include: i) bromine -containing olefins; ii) iodine-containing olefins; iii) bromine-containing vinyl ethers; iv) iodine-containing vinyl ethers; v) fluorine-containing olefins having a nitrile group; vi) fluorine-containing vinyl ethers having a nitrile group; vii) 1,1,3,3,3-pentafluoropropene (2-HPFP); viii) perfluoro(2-phenoxypropyl vinyl) ether; and ix) non-conjugated dienes.
Brominated cure site monomers may contain other halogens, preferably fluorine. Examples of brominated olefin cure site monomers are CF2xe2x95x90CFOCF2CF2CF2OCF2CF2Br; bromotrifluoroethylene; 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); and others such as vinyl bromide, 1-bromo-2,2-difluoroethylene; perfluoroallyl bromide; 4-bromo-1,1,2-trifluorobutene-1; 4-bromo-1,1,3,3,4,4,-hexafluorobutene; 4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene; 6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1 and 3,3-difluoroallyl bromide. Brominated vinyl ether cure site monomers useful in the invention include 2-bromo-perfluoroethyl perfluorovinyl ether and fluorinated compounds of the class CF2Br-Rfxe2x80x94Oxe2x80x94CFxe2x95x90CF2(Rf is a perfluoroalkylene group), such as CF2BrCF2Oxe2x80x94CFxe2x95x90CF2, and fluorovinyl ethers of the class ROCFxe2x95x90CFBr or ROCBrxe2x95x90CF2(where R is a lower alkyl group or fluoroalkyl group) such as CH3OCFxe2x95x90CFBr or CF3CH2OCFxe2x95x90CFBr.
Suitable iodinated cure site monomers include iodinated olefins of the formula: CHRxe2x95x90CHxe2x80x94Zxe2x80x94CH2CHRxe2x80x94I, wherein R is xe2x80x94H or xe2x80x94CH3; Z is a C1-C18 (per)fluoroalkylene radical, linear or branched, optionally containing one or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radical as disclosed in U.S. Pat. No. 5,674,959. Other examples of useful iodinated cure site monomers are unsaturated ethers of the formula: I(CH2CF2CF2)nOCFxe2x95x90CF2 and ICH2CF2O[CF(CF3)CF2O]nCFxe2x95x90CF2, and the like, wherein n=1-3, such as disclosed in U.S. Pat. No. 5,717,036. In addition, suitable iodinated cure site monomers including iodoethylene, 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); 3-chloro-4-iodo-3,4,4-trifluorobutene; 2-iodo -1,1,2,2-tetrafluoro-1-(vinyloxy)ethane; 2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene; 1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethyl vinyl ether; 3,3,4,5,5,5-hexafluoro-4-iodopentene; and iodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyl iodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also useful cure site monomers.
Useful nitrile-containing cure site monomers include those of the formulas shown below.
CF2xe2x95x90CFxe2x80x94O(CF2)nxe2x80x94CNxe2x80x83xe2x80x83(VI)
where nxe2x95x902-12, preferably 2-6;
CF2xe2x95x90CF-O[CF2xe2x80x94CF(CF3)xe2x80x94O]nxe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94CNxe2x80x83xe2x80x83(VII)
where n=0-4, preferably 0-2;
CF2xe2x95x90CFxe2x80x94[OCF2CF(CF3)]xxe2x80x94Oxe2x80x94(CF2)nxe2x80x94CNxe2x80x83xe2x80x83(VIII)
where x=1-2, and n=1-4; and
CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2)nxe2x80x94Oxe2x80x94CF(CF3)CNxe2x80x83xe2x80x83(IX)
where n=2-4. Those of formula (VIII) are preferred. Especially preferred cure site monomers are perfluorinated polyethers having a nitrile group and a trifluorovinyl ether group. A most preferred cure site monomer is
CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2CNxe2x80x83xe2x80x83(X)
i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.
Examples of non-conjugated diene cure site monomers include, but are not limited to 1,4-pentadiene; 1,5-hexadiene; 1,7-octadiene; 3,3,4,4-tetrafluoro-1,5-hexadiene; and others, such as those disclosed in Canadian Patent 2,067,891 and European Patent 0784064A1. A suitable triene is 8-methyl-4-ethylidene-1,7-octadiene.
Of the cure site monomers listed above, preferred compounds, for situations wherein the fluoroelastomer will be cured with peroxide, include 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); allyl iodide; bromotrifluoroethylene and 8-CNVE. When the fluoroelastomer will be cured with a polyol, 2-HPFP or perfluoro(2-phenoxypropyl vinyl) ether is the preferred cure site monomer. When the fluoroelastomer will be cured with a tetraamine, bis(aminophenol) or bis(thioaminophenol), 8-CNVE is the preferred cure site monomer.
Units of cure site monomer, when present in the fluoroelastomers manufactured by the process of this invention, are typically present at a level of 0.05-10 wt. % (based on the total weight of fluoroelastomer), preferably 0.05-5 wt. % and most preferably between 0.05 and 3 wt. %.
Specific fluoroelastomers which may be produced by the process of this invention include, but are not limited to those having at least 58 wt. % fluorine and comprising copolymerized units of i) vinylidene fluoride and hexafluoropropylene; ii) vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene; iii) vinylidene fluoride, hexafluoropropylene, tetrafluoroethylene and 4-bromo-3,3,4,4-tetrafluorobutene-1; iv) vinylidene fluoride, hexafluoropropylene, tetrafluoroethylene and 4-iodo-3,3,4,4-tetrafluorobutene-1; v) vinylidene fluoride, perfluoro(methyl vinyl) ether, tetrafluoroethylene and 4-bromo-3,3,4,4-tetrafluorobutene-1; vi) vinylidene fluoride, perfluoro(methyl vinyl) ether, tetrafluoroethylene and 4-iodo-3,3,4,4-tetrafluorobutene-1; vii) vinylidene fluoride, perfluoro(methyl vinyl) ether, tetrafluoroethylene and 1,1,3,3,3-pentafluoropropene; viii) tetrafluoroethylene, perfluoro(methyl vinyl) ether and ethylene; ix) tetrafluoroethylene, perfluoro(methyl vinyl) ether, ethylene and 4-bromo-3,3,4,4-tetrafluorobutene-1; x) tetrafluoroethylene, perfluoro(methyl vinyl) ether, ethylene and 4-iodo-3,3,4,4-tetrafluorobutene-1; xi) tetrafluoroethylene, propylene and vinylidene fluoride; xii) tetrafluoroethylene and perfluoro(methyl vinyl) ether; xiii) tetrafluoroethylene, perfluoro(methyl vinyl) ether and perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene); xiv) tetrafluoroethylene, perfluoro(methyl vinyl) ether and 4-bromo-3,3,4,4-tetrafluorobutene-1; xv) tetrafluoroethylene, perfluoro(methyl vinyl) ether and 4-iodo-3,3,4,4-tetrafluorobutene-1; and xvi) tetrafluoroethylene, perfluoro(methyl vinyl) ether and perfluoro(2-phenoxypropyl vinyl) ether.
Additionally, iodine-containing endgroups, bromine-containing endgroups or mixtures thereof may optionally be present at one or both of the fluoroelastomer polymer chain ends as a result of the use of chain transfer or molecular weight regulating agents during preparation of the fluoroelastomers. The amount of chain transfer agent, when employed, is calculated to result in an iodine or bromine level in the fluoroelastomer in the range of 0.005-5 wt. %, preferably 0.05-3 wt. %.
Examples of chain transfer agents include iodine-containing compounds that result in incorporation of bound iodine at one or both ends of the polymer molecules. Methylene iodide; 1,4-diiodoperfluoro-n-butane; and 1,6-diiodo-3,3,4,4-tetrafluorohexane are representative of such agents. Other iodinated chain transfer agents include 1,3-diiodoperfluoropropane; 1,6-diiodoperfluorohexane; 1,3-diiodo-2-chloroperfluoropropane; 1,2-di(iododifluoromethyl)perfluorocyclobutane; monoiodoperfluoroethane; monoiodoperfluorobutane; 2-iodo-1-hydroperfluoroethane, etc. Also included are the cyano-iodine chain transfer agents disclosed European Patent 0868447A1. Particularly preferred are diiodinated chain transfer agents.
Examples of brominated chain transfer agents include 1-bromo-2-iodoperfluoroethane; 1-bromo-3-iodoperfluoropropane; 1-iodo-2-bromo-1,1-difluoroethane and others such as disclosed in U.S. Pat. No. 5,151,492.
Other chain transfer agents suitable for use in the process of this invention include those disclosed in U.S. Pat. No. 3,707,529. Examples of such agents include isopropanol, diethylmalonate, ethyl acetate, carbon tetrachloride, acetone and dodecyl mercaptan.
Cure site monomers and chain transfer agents may be added to the reactor neat or as solutions. In addition to being introduced into the reactor near the beginning of polymerization, quantities of chain transfer agent may be added throughout the entire polymerization reaction period, depending upon the desired composition of the fluoroelastomer being produced, the chain transfer agent being employed, and the total reaction time.
Surprisingly, it has been found that a surfactant of the formula CH3xe2x80x94(CH2)nxe2x80x94SO3M, CH3xe2x80x94(CH2)nxe2x80x94C6H4xe2x80x94SO3M, or CH3xe2x80x94(CH2)nxe2x80x94CHxe2x95x90CHCH2xe2x80x94SO3M may be employed as the dispersing agent in the polymerization process of this invention. Surfactant of formula CH3xe2x80x94(CH2)nxe2x80x94SO3M is preferred. In these formulae n is an integer from 6 to 17, or mixtures thereof, and M is a cation having a valence of 1 (e.g. H+, Na+, K+, NH4+, etc.). Preferably n is an integer from 7 to 14. Surfactants having n values less than 6 tend to be poor soaps and poor dispersing agents for highly fluorinated fluoroelastomer latex particles. Surfactants having n values greater than 17, tend to be insufficiently soluble in water to be useful in this invention, or form insoluble salts with coagulants which are difficult to wash from fluoroelastomer crumb. The dispersing agent may also be a mixture of any two or more surfactants of the above general formulae. Specific examples of surfactants which may be employed in the emulsion polymerization process of the invention include, but are not limited to CH3(CH2)7xe2x80x94SO3Na; CH3(CH2)11C6H4xe2x80x94SO3Na; CH3xe2x80x94(CH2)8xe2x80x94CHxe2x95x90CHxe2x80x94CH2xe2x80x94SO3Na.
One skilled in the art of fluoroelastomer preparation would not predict that a hydrocarbon surfactant of the above general formulae would be capable of stabilizing highly fluorinated (i.e. containing at least 58 wt. % fluorine) fluoroelastomer latex particles. Therefore, such a surfactant would not likely be useful in an emulsion polymerization process for manufacturing highly fluorinated fluoroelastomers. Also, hydrocarbon surfactants generally act as strong chain transfer agents (due to the large number of Cxe2x80x94H bonds available for attack by radicals) and limit the molecular weight of polymers prepared in their presence. However, stable fluoroelastomer latex dispersions are surprisingly formed in the emulsion polymerization process of this invention which employs the above hydrocarbon surfactant as dispersing agent. Also, high molecular weight (i.e. having a number average molecular weight of 106 or greater) fluoroelastomers may readily be prepared in the process of this invention.
The amount of surfactant to be employed in the aqueous emulsion polymerization solution is determined by balancing emulsion stability and polymerization rate with foam generation. If too little surfactant is used, excessive reactor fouling will occur and reaction rate may be undesirably slow. If too much surfactant is used, excessive foam will be generated and it will be difficult to remove the excess surfactant from the fluoroelastomer, thus retarding the vulcanization of the fluoroelastomer with bisphenol curatives. In an emulsion polymerization process of this invention, the amount of surfactant employed is typically 0.05 to 3 wt. %, based on the total weight of fluoroelastomer being produced. The preceding amounts are based on the amount of active ingredient, not on amount of a surfactant solution containing less that 100% active ingredient.
The emulsion polymerization process of this invention may be a continuous, semi-batch or batch process.
In the semi-batch emulsion polymerization process of this invention, a gaseous monomer mixture of a desired composition (initial monomer charge) is introduced into a reactor which contains an aqueous solution. The reactor is typically not completely filled with the aqueous solution, so that a vapor space remains. The aqueous solution comprises a hydrocarbon sulfonate surfactant dispersing agent of the type discussed above. Optionally, the aqueous solution may contain a pH buffer, such as a phosphate or acetate buffer for controlling the pH of the polymerization reaction. Instead of a buffer, a base, such as NaOH may be used to control pH. Generally, pH is controlled to between 1 and 7 (preferably 3-7), depending upon the type of fluoroelastomer being prepared. Alternatively, or additionally, pH buffer or base may be added to the reactor at various times throughout the polymerization reaction, either alone or in combination with other ingredients such as polymerization initiator, liquid cure site monomer, additional hydrocarbon sulfonate surfactant or chain transfer agent. Also optionally, the initial aqueous solution may contain a water-soluble inorganic peroxide polymerization initiator. In addition, the initial aqueous solution may contain a nucleating agent, such as a fluoroelastomer seed polymer prepared previously, in order to promote fluoroelastomer latex particle formation and thus speed up the polymerization process.
The initial monomer charge contains a quantity of a first monomer of either TFE or VF2 and one or more additional monomers which are different from the first monomer. The amount of monomer mixture contained in the initial charge is set so as to result in a reactor pressure between 0.5 and 10 MPa.
The monomer mixture is dispersed in the aqueous medium and, optionally, a chain transfer agent may also be added at this point while the reaction mixture is agitated, typically by mechanical stirring. In the initial gaseous monomer charge, the relative amount of each monomer is dictated by reaction kinetics and is set so as to result in a fluoroelastomer having the desired ratio of copolymerized monomer units (i.e. very slow reacting monomers must be present in a higher amount relative to the other monomers than is desired in the composition of the fluoroelastomer to be produced).
The temperature of the semi-batch reaction mixture is maintained in the range of 25xc2x0 C.-130xc2x0 C., preferably 50xc2x0 C.-100xc2x0 C. Polymerization begins when the initiator either thermally decomposes or reacts with reducing agent and the resulting radicals react with dispersed monomer.
Additional quantities of the gaseous major monomers and cure site monomer (incremental feed) are added at a controlled rate throughout the polymerization in order to maintain a constant reactor pressure at a controlled temperature. The relative ratio of monomers contained in the incremental feed is set to be approximately the same as the desired ratio of copolymerized monomer units in the resulting fluoroelastomer. Thus, the incremental feed contains between 25 to 70 weight percent, based on the total weight of the monomer mixture, of a first monomer of either TFE or VF2 and 75 to 30 weight percent of one or more additional monomers that are different from the first monomer. Chain transfer agent may also, optionally, be introduced into the reactor at any point during this stage of the polymerization. Typically, additional polymerization initiator and hydrocarbon sulfonate surfactant are also fed to the reactor during this stage of polymerization. The amount of polymer formed is approximately equal to the cumulative amount of incremental monomer feed. One skilled in the art will recognize that the molar ratio of monomers in the incremental feed is not necessarily exactly the same as that of the desired (i.e. selected) copolymerized monomer unit composition in the resulting fluoroelastomer because the composition of the initial charge may not be exactly that required for the selected final fluoroelastomer composition, or because a portion of the monomers in the incremental feed may dissolve into the polymer particles already formed, without reacting. Polymerization times in the range of from 2 to 30 hours are typically employed in this semi-batch polymerization process.
The continuous emulsion polymerization process of this invention differs from the semi-batch process in the following manner. The reactor is completely filled with aqueous solution so that there is no vapor space. Gaseous monomers and solutions of other ingredients such as water-soluble monomers, chain transfer agents, buffer, bases, polymerization initiator, surfactant, etc., are fed to the reactor in separate streams at a constant rate. Feed rates are controlled so that the average polymer residence time in the reactor is generally between 0.2 to 4 hours. Short residence times are employed for reactive monomers, whereas less reactive monomers such as perfluoro(alkyl vinyl) ethers require more time. The temperature of the continuous process reaction mixture is maintained in the range of 25xc2x0 C.-130xc2x0 C., preferably 80xc2x0 C.-120xc2x0 C. Also, fluoroelastomer latex particles are more readily formed in the continuous process so that a nucleating agent is not typically required in order to start the polymerization reaction.
In the process of this invention, the polymerization temperature is maintained in the range of 25xc2x0-130xc2x0 C. If the temperature is below 25xc2x0 C., the rate of polymerization is too slow for efficient reaction on a commercial scale, while if the temperature is above 130xc2x0 C., the reactor pressure required in order to maintain polymerization is too high to be practical.
The polymerization pressure is controlled in the range of 0.5 to 10 MPa, preferably 1 to 6.2 MPa. In a semi-batch process, the desired polymerization pressure is initially achieved by adjusting the amount of gaseous monomers in the initial charge, and after the reaction is initiated, the pressure is adjusted by controlling the incremental gaseous monomer feed. In a continuous process, pressure is adjusted by a back-pressure regulator in the dispersion effluent line. The polymerization pressure is set in the above range because if it is below 1 MPa, the monomer concentration in the polymerization reaction system is too low to obtain a satisfactory reaction rate. In addition, the molecular weight does not increase sufficiently. If the pressure is above 10 MPa, the cost of the required high pressure equipment is very high.
The amount of fluoroelastomer copolymer formed is approximately equal to the amount of incremental feed charged, and is in the range of 10-30 parts by weight of copolymer per 100 parts by weight of aqueous medium, preferably in the range of 20-25 parts by weight of the copolymer. The degree of copolymer formation is set in the above range because if it is less than 10 parts by weight, productivity is undesirably low, while if it is above 30 parts by weight, the solids content becomes too high for satisfactory stirring.
Water-soluble peroxides which may be used to initiate polymerization in this invention include, for example, the ammonium, sodium or potassium salts of hydrogen persulfate. In a redox-type initiation, a reducing agent such as sodium sulfite, is present in addition to the peroxide. These water-soluble peroxides may be used alone or as a mixture of two or more types. The amount to be used is selected generally in the range of 0.01 to 0.4 parts by weight per 100 parts by weight of polymer, preferably 0.05 to 0.3. During polymerization some of the fluoroelastomer polymer chain ends are capped with fragments generated by the decomposition of these peroxides.
Optionally, fluoroelastomer gum or crumb may be isolated from the fluoroelastomer dispersions produced by the process of this invention by the addition of a coagulating agent to the dispersion. Any coagulating agent known in the art may be used. Preferably, a coagulating agent is chosen which forms a water-soluble salt with the surfactant contained in the dispersion. Otherwise, precipitated surfactant salt may become entrained in the isolated fluoroelastomer and then retard curing of the fluoroelastomer with bisphenol-type curatives.
In one isolation process, the fluoroelastomer dispersion is adjusted to a pH less than 4 and then coagulated by addition of an aluminum salt. Undesirable insoluble aluminum hydroxides form at pH values greater than 4. Aluminum salts useful as coagulating agents include, but are not limited to aluminum sulfate and alums of the general formula M""A1(SO4)2. 12H2O, wherein Mxe2x80x2 is a univalent cation, other than lithium. The resulting coagulated fluoroelastomer may then be filtered, washed and dried.
In addition to aluminum salts, common coagulants such as calcium salts (e.g. calcium nitrate) or magnesium salts (e.g. magnesium sulfate), and some salts of univalent cations (e.g. sodium chloride or potassium chloride), may be used to coagulate fluoroelastomers produced in a process employing a surfactant that forms a water soluble salt with such coagulants.
Another aspect of this invention is the curable fluoroelastomers that are produced by the process of this invention. Such fluoroelastomers are generally molded and then vulcanized during fabrication into finished products such as seals, wire coatings, hose, etc. Suitable vulcanization methods employ, for example, polyol, polyamine, organic peroxide, organotin, bis(aminophenol), tetraamine, or bis(thioaminophenol) compounds as curatives.
The fluoroelastomers prepared by the process of this invention are useful in many industrial applications including seals, wire coatings, tubing and laminates.